What do I need to get to upgrade 10 speed Campagnolo to 11 speed for a bike to be used on a trainer only?

The simple, cheap way to do this using an off-the-shelf product is buy a 10-speed Campy to HG conversion cassette. Various options for this have existed over time; the only one I can see now is an IRD 11-32, which would work if you used the low limit to lock out whatever cogs on it the RD can't clear or that you don't have the chain length for. This is probably the way to go if you can stand the gearing jumps being larger than you're used to.

There is no technical reason why spacers for fully interchangeable/customizable cassette systems such as the Miche one couldn't be made to do this, and if they did exist then that would probably be the cheapest. I don't think they exist commercially, though. It may be possible to make your own with off-the-shelf spacers and microshims, once you figured out what the thickness needed to be.

To make the bike 11-speed, the answer is the chain and right shifter for sure, the front derailleur if you want good front shifting and minimal chance of chain drop, and then potentially the rear derailleur. The RD actuation ratio between Campy 10 and 11 didn't change but the capacity and large cog clearance numbers did in some cases, and you haven't told us your cassette range. If you then put a Campy 11 cassette on the wheel too, drop-in wheel interchangeability probably won't work between your Campy wheel and your trainer if you do this with its existing cassette unless you space out whichever is more inboard, at which point drop-in interchangeability won't work between various wheels if it's the wheel cassette that needs spacing, or between the trainer and different bikes if it's the trainer cassette.

To my awareness there's no right Ergo you can buy here that doesn't leave you with a tactile mismatch. 11-speed index rings that fit into your right shifter were never released.